As a technique of removing nitrogen oxides (which will be hereinafter referred to as “NOx”) contained in the exhaust gas of an internal combustion engine, it has been known to provide an NOx storage reduction catalyst (which will be hereinafter referred to as an “NOx catalyst”) in the exhaust system of the internal combustion engine. Since the NOx removing ability of the NOx catalyst is deteriorated as the amount of NOx stored in the NOx catalyst increases, reduction of NOx is executed by supplying an reducing agent to the NOx catalyst and making the air-fuel ratio of the exhaust gas flowing into the NOx catalyst rich. (Such a process will be hereinafter referred to as the “NOx reduction process”.)
In the NOx catalyst are also stored sulfur oxides (which will be hereinafter referred to as “SOx”) contained in the exhaust gas according to a mechanism similar to the case of NOx. As the amount of SOx stored in the NOx catalyst increases, SOx poisoning that causes a decrease in the NOx storing ability of the NOx catalyst can occur in some cases. If it occurs, reduction of the SOx stored in the NOx catalyst is executed by raising the temperature of the NOx catalyst (e.g. up to 600° C.-650° C.) and supplying a reducing agent to the NOx catalyst. (Such a process will be hereinafter referred to as the “SOx poisoning elimination process”.)
When the NOx reduction process or SOx poisoning elimination process (which will be collectively referred as the “performance regeneration process”, hereinafter) is executed on an NOx catalyst, if the intake air quantity of the internal combustion engine is small, the quantity of the reducing agent flowing into the NOx catalyst can be insufficient in some cases even if the air-fuel ratio of the exhaust gas flowing into the NOx catalyst is made rich.
In connection with this, Japanese Patent Application Laid-Open No. 2002-130008 discloses a technique in which when the intake air quantity of the internal combustion engine is insufficient, execution of the performance regeneration process is disabled in order to prevent efficiency of the performance regeneration process from being deteriorated.
However, in the case where the performance regeneration process is executed on an NOx catalyst at a time when the intake air quantity is larger, the flow rate of the exhaust gas flowing in the exhaust passage is also larger, and accordingly the quantity of reducing agent required to be supplied to make the air-fuel ratio of the exhaust gas flowing into the NOx catalyst rich becomes larger. Therefore, fuel economy can be deteriorated in the case where fuel is used as the reducing agent, or the reducing agent can slip through the NOx catalyst and be emitted to the atmosphere, in some cases.
Japanese Patent Application Laid-Open No. 2003-20982 discloses a technique in which reducing agent is supplied to an NOx catalyst in a state where a throttle valve is closed while the internal combustion engine is decelerating and fuel-cut is being executed.
Japanese Patent Application Laid-Open No. 2004-68785 discloses a technique of performing rich spike control in which fuel is injected through a fuel injection valve provided in a cylinder when main fuel injection for combustion in the internal combustion engine is suspended during deceleration of the internal combustion engine to make the air-fuel ratio of the exhaust gas flowing into an NOx catalyst temporarily rich. According to this technique, the fuel injection pressure at the time when fuel is injected through the fuel injection valve in the rich spike control is designed to be higher than that in the main fuel injection, with a view to remove NOx with reliability.
In some exhaust gas purification systems, the exhaust passage of the internal combustion engine branches into two branch passages arranged in parallel each of which is provided with an NOx catalyst, and the flow rate of the exhaust gas flowing in each branch passage can be changed. In such systems, when the performance regeneration process is executed on the NOx catalyst disposed in one of the branch passages, the flow rate of the exhaust gas that flows into that branch passage is made small with a view to enhance the efficiency of the performance regeneration of the NOx catalyst. (Such process of making the flow rate of the exhaust gas small will be hereinafter referred to as the “low SV control”.)